1. Field of the Invention
Embodiments of the invention described herein pertain to the field of medical apparatus and methods. More particularly, but not by way of limitation, one or more embodiments of the invention generally relate to a heart stimulator for pacing at least an atrium of a heart by delivery of electrical stimulation pulses to the atrium and more specifically relate to an atrial heart stimulator that provides automatic atrial capture threshold testing.
2. Description of the Related Art
Typically, an atrial heart stimulator is an implantable dual-chamber pacemaker providing a stimulation pulse generator for generating atrial stimulation pulses to be delivered to an atrium of a heart and ventricular stimulation pulses to be delivered to a ventricle of a heart. In general, a dual chamber pacemaker provides a separate stimulation pulse generator for each heart chamber to be stimulated. However, the use of a single switchable stimulation pulse generator is also possible.
Heart stimulators like pacemakers deliver stimulation pulses (also called pacing pulses) to the heart to cause the stimulated heart chamber to contract if no natural (intrinsic) contraction of the heart chamber occurs. A stimulation pulse for delivery to the myocardium (heart tissue) of the atrium of a heart is called an atrial stimulation pulse or an atrial pace Ap. Likewise, a stimulation pulse for delivery to a ventricle of a heart is called a ventricular stimulation pulse or a ventricular pace Vp.
In order to enable the heart stimulator to detect intrinsic contractions of a heart chamber, a demand pacemaker usually comprises a sensing channel comprising sense amplifiers for processing electrical signals originating from a respective heart chamber in order to detect electrical signals corresponding to a depolarization of the heart tissue that is followed by a natural contraction of the heart.
Atrial and ventricular stimulation pulse generators and atrial and ventricular sensing channels are connected to or are connectable to electrode leads having electrodes for placement in a respective heart chamber. The electrodes connected to a stimulation pulse generator serve for delivery of stimulation pulses to the heart tissue whereas the electrodes connected to a respective sensing channel serve for picking up electrical signals from the heart tissue.
A control unit triggers the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses.
Depending on the mode of operation, a pacemaker delivers a stimulation pulse (pacing pulse) to a heart chamber (atrium or ventricle) only if needed, that is, if no natural excitation of that chamber occurs. Such a mode of operation is called an inhibited or demand mode of operation since the delivery of a stimulation pulse is inhibited if a natural excitation of the heart chamber is sensed within a predetermined time interval (usually called escape interval) so the heart chamber is only stimulated if demanded.
In the demand mode, the pacemakers monitors the heart chamber to be stimulated in order to determine if a cardiac excitation (heartbeat) has naturally occurred. Such natural (non-stimulated) excitation, also referred to as “intrinsic” or “sinus” cardiac activity, are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms. The recognizable electrical signals that accompany the depolarization or excitation of a heart chamber are picked up (sensed) by the atrial or the ventricular sensing channel, respectively.
A natural contraction of a heart chamber can be detected by evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a signal known as “P-wave”. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a signal known as “R-wave”. A P-wave or an R-wave signal represents an atrial event or a ventricular event, respectively, in the further course of this application.
In a demand mode of operation, the pacemaker monitors the heart for the occurrence of P-waves and/or R-waves. If such signals are sensed within a prescribed time period or time window, which is called atrial or ventricular escape interval, respectively, then the escape interval is reset (i.e., restarted) and generation of a stimulation pulse is inhibited and no unnecessary stimulation pulse is triggered. The escape interval is measured from the last heartbeat, i.e., from the last occurrence of an intrinsic (sensed) atrial event (P-wave, A-sense, AS) if the atrium is monitored, or an intrinsic (sensed) ventricular event (R-wave, V-sense, VS) if the ventricle is monitored, or the generation of a stimulation pulse (V-pace, VP; A-pace, AP) if no respective intrinsic event has occurred. If the escape interval “times-out”, i.e., if a time period equal to the escape interval has elapsed without the sensing of a P-wave and/or R-wave (depending upon which chamber of the heart is being monitored), then a stimulation pulse is triggered at the conclusion of the escape interval. In this way, the pacemaker provides stimulation pulses “on demand,” i.e., only as needed, when intrinsic cardiac activity does not occur within the prescribed escape interval.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. According to this code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized:                (1) an Inhibiting (“I”) response wherein a stimulation pulse is triggered for the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited;        (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart is triggered a prescribed period of time after a sensed event (i.e. triggering of a ventricular stimulation pulse at the end of an AV-interval that is started (Triggered) by an atrial sense or pacing event); or        (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other, i.e. when an atrial sense event inhibits triggering of an atrial stimulation pulse at the end of an atrial escape interval and at the same time starts (triggers) a next ventricular and atrial escape interval.        
An additional fourth letter “R” may be added to the basic three letter code to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
Multiple-mode, demand-type, cardiac pacemakers allow a sequence of contractions of the heart's chamber which equals as far as possible a natural behavior of the healthy heart for damaged or diseased hearts that are unable to do so on their own.
In a healthy heart, initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall and acts as a natural “pacemaker” of the heart. In a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. The natural stimulus causing the atrium to contract is conducted to ventricle via the atrioventricular node (AV node) with a short, natural delay, the atrioventricular delay (AV-delay). Thus a short time after an atrial contraction (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 180 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. Thus, in a healthy heart providing proper AV− synchrony an atrial contraction (A) is followed a relatively short time thereafter by a ventricle contraction (V), that in turn is followed a relatively long time thereafter by the next atrial contraction and so on. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump.
To mimic the natural behavior of a heart, a dual-chamber pacemaker, in conventional manner, defines a basic atrial escape interval (AEI) that sets the time interval for scheduling an atrial stimulation pulse. The atrial escape interval can be started by a ventricular event and end with an atrial event. A basic AV delay (AVD) or ventricular escape interval (VEI) sets the time interval or delay between an atrial event and a ventricular event. In such embodiment, AEI and AVD (or VEI) thus together define a length of a heart cycle which is reciprocal to the pacing rate at which stimulation pulses are generated and delivered to a patient's heart in the absence of sensed natural cardiac activity.
For the purpose of this application, a “ventricular event” (V) may refer either a natural ventricular excitation (intrinsic ventricular event; Vs) which is sensed as an R-wave or a ventricular stimulation pulse (V-pulse, Vp). Similarly, an atrial event (A) shall refer to both, a P-wave (A-sense; As) or an atrial stimulation pulse (A-pulse, Ap).
Dual chamber pacing often times is in a DDD mode of operation of the pacemaker.
In general, there are two kinds of dual-chamber DDD-timing schemes:
In atrial-based DDD pacing, all timing is controlled either from the sensing of atrial activity (a P-wave; A-sense; As) or an atrial pacing (Ap). When a P-wave (Vs) is sensed, two separate timers are started that operate in parallel. A first timer defines an atrial escape interval (A-Ap), which, if timed-out, results in an atrial paced event (Ap). A second timer defines a separate AV delay, which, if timed-out, results in a ventricular paced event (Vp). The first and second timers both start upon sensed or paced atrial activity (A). The AV delay timer does not affect the basic atrial escape interval timer. The atrial escape interval timer thus controls the basic functioning rate of the pacemaker from atrial to atrial event. The ventricle is paced, if needed, at a rate that tracks the sensed atrial rate. If no atrial activity is sensed, then the atrium is also paced at a rate equal to the set rate. Nonetheless, even when operating in such atrial-based mode, there still remains a need to enhance pacemaker longevity, as well as a need to allow the heart to beat at its own rhythm more often.
In atrial based DDD-pacing the atrial escape interval usually is an A-A-interval which is simultaneously started with a ventricular escape interval (AV-interval)
As an alternative to above-described atrial-based DDD pacing, there is also a second type of dual chamber operation known as ventricular-based pacing. In ventricular-based DDD pacing (sometimes referred to as ventricular-based timing), two parallel timers are used, as indicated above. By a ventricular event, a VA Delay timer is started. If the VA Delay timer times out all the way, an atrial pulse (Ap) is provided. Thus, the VA-delay timer defines an atrial escape interval (AEI). If a P-wave is sensed before the VA Delay timer times-out, such sensing terminates the VA Delay timer. The sensing of a P-wave or the generating of an A-pulse thus defines an atrial event. A ventricular event also starts a Ventricular Escape Interval timer. If this Ventricular Escape Interval timer times-out all the way, a ventricular pulse (Vp) is provided. If an R-wave is sensed before the Ventricular Escape Interval timer times-out, such sensing terminates the Ventricular Escape Interval Timer. The sensing of an R-wave (V-sense; Vs) or the generating of ventricular pulse thus comprise a ventricular event (V), and this ventricular event (V) starts both the VA Delay timer and the Ventricular Escape Interval Timer again.
In patients having an AV-block, the natural conduction from the atrium to the ventricle is affected. However, the atrium itself may contract in a natural way with a physiologically adequate rate. In the DDD(R) mode of operation an AV-sequential stimulation or atrium-synchronous pacing is possible, which allows tracking of intrinsic atrial contractions and to stimulate the ventricle with an (artificial) AV delay after each sensed atrial contraction in order to maintain AV synchronicity. In such mode of operation the maximum AV delay between an atrial event and the next paced ventricular event is given by the ventricular escape interval.
The choice of an adequate duration of an escape interval depends at least on two demands: the escape interval shall reflect the natural timing of a healthy heart. Therefore, the ventricular escape interval would be chosen to match the natural atrioventricular conduction time in a healthy heart. On the other hand, it is an object to allow as many natural contractions of a heart chamber as possible. Therefore, timeout of the escape interval should not occur too early to give the heart the chance to contract on its own.
To meet these demands one of the programmable modes that has been used with programmable pacemakers for many years is a mode known as the “hysteresis” mode. The hysteresis mode is used in conjunction with selected other modes, such as single-chamber demand pacing, to allow the natural sinus rhythm of the heart to persist at rates less than the programmed minimum rate of the pacemaker. The programmed minimum rate of the pacemaker, in turn, sets the atrial escape interval. During pacing, i.e., during those times when the pacemaker is generating stimulation pulses, the pacemaker thus stimulates the heart at the rate set by the atrial escape interval or the sum of the atrial escape interval and the AV delay, respectively, i.e., upon the timing-out of each atrial and/or ventricular escape interval. When the hysteresis mode is enabled, sensed cardiac activity causes the pacemaker escape interval to be extended, or lengthened, thereby providing a longer period of time within which natural cardiac activity may occur before the pacemaker steps in to generate a stimulation pulse. Should the intrinsic rate of the heart fall below the programmed hysteresis rate, i.e., should no intrinsic cardiac activity be sensed during the lengthened escape interval, then a stimulation pulse is generated, and the escape interval reverts back to its initial value, as determined by the programmed minimum rate.
Further intervals set to determine the pacemaker's behavior include refractory periods like a post ventricular refractory period (PVARP), which is started with delivery of a ventricular pacing pulse and during which no atrial activity is sensed thus rendering the pacemaker refractory (insensitive) in the atrium during PVARP. This interval and other intervals will not be discussed further herein since these intervals are understood by one skilled in the art.
A stimulation pulse to the myocardium only causes capture of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse strength is above the capture threshold of said myocardium. A sub-threshold stimulation pulse will not cause capture even if delivered to the myocardium in its non-refractory state. Capture only occurs if a stimulation pulse is strong enough to cause excitation of the myocardium. Pulse strength depends both on duration and amplitude of an electrical stimulation pulse. Usually, stimulation pulse strength is adjusted by adjusting the pulse amplitude while maintaining the pulse duration.
The stimulation pulse strength just enough to cause capture of a heart chamber is called capture threshold. It is desirable to adjust the stimulation pulse strength so that the stimulation pulse strengths for particular heart chambers just above capture threshold in order to spend as little energy as possible for a single stimulation pulse while ensuring reliable effectiveness of a stimulation pulse delivered.
Since capture threshold may vary from heart chamber to heart chamber and from patient to patient and may even very over time, there is a need for automatic capture testing and determination in particular as far as implantable heart stimulators are concerned.
In order to determine whether a stimulation pulse has led to capture or not, the number of approaches are used. One approach is to detect characteristic signal pattern in an electrocardiogram originating from a particular heart chamber wherein the characteristic signal pattern corresponds to an evoked response. An alternative approach is to time the delivery of the stimulation pulse such that the stimulation pulse would precede an intrinsic contraction of the heart chamber by a small amount of time. Such stimulation pulse renders the myocardium of the heart chamber refractory. In its refractory period, the heart chamber is not sensitive to any stimulus or intrinsic excitation. Therefore, a stimulation pulse delivered just before a natural contraction of the heart chamber occurs should suppress the natural contraction of the heart chamber. On the other hand, if the stimulation pulse is ineffective, one could sense a natural contraction of the heart chamber shortly after delivery of said stimulation pulse.
In a rhythm based capture detection as for example described in US Patent application 2005/0222630, for purpose of atrial capture detection the atrium is stimulated with an overdrive pacing rate which is higher than the intrinsic atrial rate for a predetermined number of heart cycles. Capture is detected when the number of sensed intrinsic atrial events is smaller than effort of the predetermined number of heart cycles stimulated with the overdrive pacing rate. Loss of capture is detected when the number of sensed atrial events within a set period of time of stimulating the atrium with the overdrive pacing rate is larger than the number of stimulated atrial heart cycles stimulated with the overdrive pacing rate.
For at least the limitations described above there is a need for an atrial heart stimulator apparatus and method.